Refractive index and thermo-optic coefficient of Ge-Sb-Se chalcogenide glass

Abstract

Ge-Sb-Se chalcogenide glass is environmentally friendly, and has wide infrared transmitting window, high optical nonlinearity, as well as good mechanical property. These make it a good material for infrared transmission and nonlinear optics. In optical designs, the refractive index (<i>n</i>) and thermo-optic coefficient (<i>ζ</i>) of the glass are key technical parameters. In order to predict and tailor the <i>n</i> and <i>ζ</i> of Ge-Sb-Se glass, compositions with different chemical and topological features are prepared, their <i>n</i>, <i>ζ</i>, density (<i>d</i>) and volume expansion coefficient (<i>β</i>) are measured, and the composition dependence of the parameters is systematically investigated. The chemical feature of the glass is quantified by the percentage deviation of the composition from the stoichiometric ratio and denoted as <i>dSe</i>. The topological feature is represented by the mean coordination number <<i>r</i>> of each atom in the composition. It is shown that the <i>n</i> of Ge-Sb-Se glass increases with <i>d</i> increasing; the <i>ζ</i> decreases almost linearly with <i>β</i> increasing; and the <i>β</i> decreases with <i>dSe</i> decreasing or <<i>r</i>> increasing. When the Ge content is fixed, the <i>d</i> increases with <i>dSe</i> decreasing or <<i>r</i>> increasing; when the Sb concentration is fixed, the <i>d</i> has a minimum value at <i>dSe</i>=0. Based on the measured <i>d</i> and <i>n</i>, the molar refractivity (<i>R</i>_i) of Ge, Sb and Se elements in a spectral range of 2-12 μm are calculated. The obtained value of <i>R</i>_Ge is in a range of 10.16-10.50 cm³/mol, <i>R</i>_Sd in a range of 16.71-17.08 cm³/mol, and <i>R</i>_Se in a range of 11.15-11.21 cm³/mol. When the <i>R</i>_i and <i>d</i> are used to compute <i>n</i> of any composition, the discrepancy between the calculated value and the measured one is less than 1%. According to the measured <i>ζ</i> and <i>β</i>, the thermal coefficients of the molar refractivity (<i>φ</i>_i) of Ge, Sb, and Se elements in a wavelength region of 2-12 μm are computed. The optimal value of <i>φ</i>_Ge is in a range of 21.1-22.6 ppm/K, <i>φ</i>_Sb in a range of 7.2-8.4 ppm/K, and <i>φ</i>_Se in a range of 90.2-94.2 ppm/K. When the <i>φ</i>_i and <i>β</i> are used to compute <i>ζ</i> of any composition, the discrepancy between the calculated value and the measured value is less than 6 ppm/K.